This invention relates generally to semiconductor optical waveguide devices and more particularly to shaping of the overall spectral response of the device by cascading a plurality of semiconductor optical amplifiers.
Fiber optic communications systems have gained widespread acceptance over the past few decades. With the advent of optical fiber, communication signals are transmitted as light propagating along a fiber supporting total internal reflection of the light propagating therein. Many communication systems rely on optical communications because they are less susceptible to noise induced by external sources and are capable of supporting very high speed carrier signals and increased bandwidth. Unfortunately, optical fiber components are bulky and often require hand assembly resulting in lower yield and higher costs. One modern approach to automating manufacture in the field of communications is integration. Integrated electronic circuits (ICs) are well known and their widespread use in every field is a clear indication of their cost effectiveness and robustness. A similar approach to optical communication components could prove very helpful.
Unfortunately, integrated optical devices are generally quite lossy. In order to compensate for the performance of a lossy device, one approach is to use optical amplifiers to amplify the light provided to the lossy component. However, because of non uniformities in optical amplification and in optical response of an integrated component, results vary and generally, this approach is not used with repeatable and easily manufacturable results.
Due to these non-uniformities, each optical component has a typical spectral response, i.e. the effect of the component upon a light signal depends upon the wavelength of the light signal. For example, an optical amplifier produces differing gains for different wavelength channels when it is pumped. Using this technology requires some means of ensuring that the different wavelength channels each receive the same amount of optical amplification. To compensate for the differing gains, a gain flattening filter is introduced to the amplifier assembly. The spectral response of this filter is used to flatten the amplification of the optical amplifier.
One way of boosting the total bandwidth of an optical network is using wavelength division multiplexing or WDM. This technology allows many different wavelength channels, each with its own signal to use the same fiber. As the need for bandwidth increases the designers of the WDM components try to add more support for more channels to their products. As more and more channels are added it becomes harder to separate them and special care has to be taken for ensuring signal quality, i.e. substantially equal intensity of the channel wavelengths and separation between channels. If they are not properly separated then they begin to inadvertently share signals. Also, as the number of channels increases the components that are needed to separate the individual channels becomes more complex and difficult to build.
In U.S. Pat. No. 5,422,968 filed Mar. 4, 1994, Hanatami et al. discuss the advantages of combining an optical demultiplexer with optical amplifiers. In this patent, the technology used for wavelength demultiplexing appears to be the thin film filter. That is, a single multiplexed signal is broken up into two output signals with differing wavelength characteristics. The authors explain the advantages of adding optical amplifiers to this system for the purposes of ensuring a proper signal to noise ratio as well as locating the amplifiers between the demultiplexing elements within the optical circuit. While this design takes advantages of optical amplifiers in a demultiplexing system it ignores the advantages of integrating the amplifiers and the wavelength dispersive elements. Additionally, the optical amplifiers are sufficiently powerful to overcome the attenuation of the other components and produce an assembly that amplifies the signals entering it. While this appears advantageous, a bulk optic assembly such as this is not practical. The components are all separate and very expensive. The combining of these components is time consuming. Additionally, the finished assembly must be sufficiently large to accommodate the optical amplifiers, the demultiplexers and the optical waveguides used to connect them.
A common method of achieving the required functionality typically relies on a hybrid integration of discrete passive devices such as an optical spectral analyzer and active devices such as amplifiers and attenuators. The search for more compact and cost efficient solutions has resulted in developing integrated planar waveguide components as disclosed, for example, in C. R. Doerr et al, Dynamic Wavelength Equalizer in Silica Using the Single-Filtered-Arm Interferometer, IEEE Photon. Technol. Lett., Vol. 11, pp. 581-583, 1999, and P. M J. Schiffer et al, Smart Dynamic Wavelength Equalizer with On-Chip Spectrum Analyzer, IEEE Photon. Technol. Lett, Vol. 12, pp. 1019-1021, 2000. In these components, the optical spectral analyzer most commonly is either the arrayed waveguide grating or waveguide echelle grating and active devices are integrated within the passive ridge waveguides, physically separating the individual wavelength channels as illustrated, for example, in E. S. Koteles, Integrated Planar Waveguide Demultiplexers for High Density WDMA applications, Proc. SPIE, 1999. As a result, a compact and inexpensive integrated component for use in WDM systems is produced.
It would be advantageous to use integration such as monolithic integration for providing integrated planar waveguide components comprising a plurality of cascaded amplifiers having a different spectral response. The combination of the different spectral responses allows the design of integrated optical components having equal gain and, furthermore, allows compensating for spectral responses of passive optical elements. This technology would be highly beneficial for modern high capacity bandwidth optical networks because it provides means for accurately designing integrated optical components having an overall spectral response ensuring channel separation as well as substantially equal intensity over a channel bandwidth.
It is, therefore, an object of the invention to provide an integrated planar waveguide device having a desired overall spectral response by using a plurality of cascaded amplifiers.
It is further an object of the invention to provide a method for compensating the spectral response of passive optical elements using the cascaded amplifiers.
In accordance with the present invention there is provided a semiconductor optical amplifier comprising:
a semiconductor waveguide structure;
a first semiconductor optical amplifier integrated in the semiconductor waveguide structure, the first semiconductor optical amplifier having a first spectral response; and,
a second semiconductor optical amplifier integrated in the semiconductor waveguide structure and coupled for optically receiving an amplified optical signal from the first semiconductor optical amplifier, the second semiconductor optical amplifier having a second other spectral response.
In accordance with the present invention there is further provided an integrated semiconductor waveguide structure comprising:
a first semiconductor optical amplifier for amplifying an optical input signal, the first semiconductor optical amplifier having a first spectral response;
a second semiconductor optical amplifier coupled for optically receiving the amplified optical input signal from the first semiconductor optical amplifier, the second semiconductor optical amplifier having a second other spectral response; and,
a demultiplexer optically coupled to the second semiconductor amplifier, the demultiplexer for separating the amplified optical input signal into a plurality of optical signals within different wavelength channels.
In accordance with an aspect of the present invention there is provided a method for designing an integrated semiconductor waveguide device comprising the steps of:
providing design parameters of an optical element of the integrated semiconductor waveguide device;
providing design parameters of a first and a second semiconductor optical amplifier optically coupled to the optical element;
determining a spectral response of the optical element in dependence upon the design parameters of the optical element;
determining a spectral response of the first and the second semiconductor optical amplifier; and,
determining an overall spectral response by multiplying the spectral response of the optical element and the spectral responses of the first and the second semiconductor optical amplifier.
Using the cascaded SOAs for optical amplification according to the invention is highly advantageous by providing a large number of design possibilities for shaping the spectral response of an integrated optical device without inducing a considerable increase in cost. Furthermore, it allows the combination with optical components having a less optimal spectral response but are easier and consequently cheaper to manufacture and use of the cascaded SOAs to produce a desired overall spectral response of the device.